Most organic light emitting diodes contain an organic emissive layer that emits light by fluorescent or phosphorescent luminescence. An organic LED generally comprises an anode, a hole source, an emissive layer (EML), an electron source and a cathode. The hole source may comprise a hole injection layer (HIL) and a hole transport layer (HTL). The electron source generally comprises an electron transport layer (ETL) and possibly an electron injection layer (EIL), as shown in FIG. 1. Some OLEDs also comprise a thin layer of LiF between the electron source and the cathode.
The EML, comprised of a host material doped with one or more luminescent dyes, provides the function of light emission produced by excitons. The excitons are formed as a result of recombination of holes and electrons in the layer.
The excitons in a fluorescent emissive layer are in a singlet excited state and, therefore, only a small percentage of excitons result in fluorescent luminescence. Excitons in a phosphorescent medium are in an excited triplet state and, theoretically, all excitons can result in phosphorescent luminescence.
Adachi et al. (U.S. Pat. No. 6,645,645) discloses a phosphorescent OLED, wherein the emissive layer is made of phenanthroline (BCP) as host material doped with fac-tris(2-phenylpyridine) iridium (Ir(ppy)3).
Baldo et al. U.S. Pat. No. 6,097,147) discloses another OLED wherein the host material for the emission layer is carbazole biphenyl (CBP) doped with 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) (PtOEP).
In a phosphorescent OLED, holes from the hole transport layer recombine in the emissive layer with electrons from the electron transport layer to form triplet-based excitons. The triplet-based excitons diffuse over a relatively long distance in the emissive layer before emitting light. It is possible that some of the excitons diffuse to the cathode and are quenched by the cathode, resulting in non-radiative exciton decay. In order to reduce the quenching by the cathode, a hole blocking layer is disposed between the cathode and the emissive layer. The blocking layer can be made of N,N′-diphenyl-N,N′-bis-alpha-anpthylbenzidine (NPD), CBP, aluminum tris (8-hydroxyquioline) (Alq3) and bathocuproine (BCP), for example.
Adachi et al. (“High-efficiency red electrophosphorescence device”, Appl. Phys. Lett., Vol. 78, No. 11, 12 March 2001, pp. 1622-1624) discloses a phosphorescent OLED wherein the emissive layer consists of a conductive CBP host doped with a red phosphor bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetylacetonate) (Btp2Ir(acac)) and the blocking layer is made from 2,9-dimethyl-4,7-diphenyl-phenanthroline.
Kwong et al. (High operational stability of electrophosphorescent devices”, Appl. Phys. Lett., Vol. 81, No. 1, 1 July 2002, pp. 162-164) discloses a phosphorescent OLED wherein the emissive layer is made of CBP doped with Ir(ppy)3 and the blocking layer is made from 2,2′,2″-(1,3,5- benzenetriyl) tris-[1-phenyl-1-H-benzimidazole (TPBI), aluminum (III)bis(2- methyl-8-quinolinato) triphenylsilanolate (SAlq), aluminum (III)bis(2-methyl-8-quinolinato )4-phenolate (PAlq) or aluminum (III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq).
In prior art, metal complexes are also used in a blocking layer. For example Thompson et al. (U.S. Patent Application 2003/0175553 A1) discloses that fac-tris(1-phenylpyrazolato-N,C2)iridium(III) (Ir(ppz)3) is used as an electron/exciton blocking material. Thompson et al. also uses metal complexes such as platinum (II)(2-(4′,6′-difluorophenyl) pyridinato-N,C2)(2,4-pentanedionato) (FPt, FPl(acac)); platinum (II) (2-(4′,6′-diflurophenyl)pyridinato-N,C2)(2,2,6,6,-tetramethyl-3,5-heptanedionato) (FPt2); platinum (II)(2-(4′,6′-difluorophenyl) pyridinato-N,C2)(6-methyl-2,4-heptanedionato) (FPt3); platinum (II)(2-(4′,6′-difluorophenyl) pyridinato-N,C2)(3-ethyl-2,4-pentanedionato) (FPt4); iridium-bis(4,6,-F2-phenyl-pyridinato-N,C2)-picolinate (FIr(pic)) and N,N′-meta-dicarbazoloylbenzene (mCP) as dopants in the emissive layer. Igarashi (U.S. Patent Application 2001/0134984 A1) discloses a light-emitting device wherein at least one of the organic layers comprises a transition metal complex containing a moiety which has a transition metal ion linked to two nitrogen ions in a nitrogen-containing structure. The same transition metal ion is linked to at least one nitrogen ion in another moiety. Shen et al. (“Spirobifluorene-Linked Bisanthracene: An Efficient Blue Emitter with Pronounced Thermal Stability”, Chem. Mater. 2004, 16, 930-934) discloses the use of spiro-FPA as a blue emitter in an OLED. Jacob et al. (“Ladder-Type Pentaphenylenes and Their Polymers: Efficient Blue-Light Emitters and Electron-Accepting Materials via a Common Intermediate”, J. AM. CHEM. SOC. 2004, 126, 6987-6995) discloses a plurality of ladder-type pentaphenylenes for use both hole accepting p-type materials and electron accepting n-type materials in blue OLEDs.
It has been found that when metal complexes are used as dopants with high doping concentration, most of the metal complexes appear to be self-quenching, thereby reducing the emission efficiency in the light-emitting device.